Lubsko [ˈlupskɔ] (German: Sommerfeld, Lower Sorbian: Žemŕ), formerly Zemsz, is a town in Żary County in the Lubusz Voivodeship in western Poland.

[3] It is located within Lower Lusatia, but at some times it also belonged to Silesia, e.g. under the Polish rulers Bolesław the Brave and Henry the Bearded.

The name Sommerfeld, German for "summer field", already appeared in an 1106 deed allegedly issued by margrave Henry I of Wettin, who nevertheless had died three years before.

Given in pawn several times, Emperor Charles IV of Luxembourg finally granted Sommerfeld with Lower Lusatia to the Silesian Piast duke Bolko II the Small.

[5] During the war of the succession of the Duchy of Głogów, the town returned under Polish rule and in 1464 it paid homage and swore allegiance to Piast Duke John II the Mad.